Hello Mikey,
There are several different approaches to how a change over systems works. But regardless of the system, if the output pressure from the regulators does not comply with specification of the installation it is illegal.
So for example, it is illegal to use a 37mBar regulator or a 28mBar regulator on a caravan that is designed and tested for 30mBar which is the current standard.
Note I am only referring to the legality of the system.
However, the NCC requirement for touring caravans calls for a bulkhead fitted 30mBar regulator, so the use of bottle top regulators even of they are set to the correct pressure does not comply with the NCC guidelines.
If the change over device does not contravene the gas pressure regulations, then the dealer may fit it.